In a chain to be used for power transmission, stretch in a longitudinal direction occurs, which is caused by friction generated between members at a rotary joint portion of the chain due to operations of the chain over years. This stretch appears as a phenomenon in which intervals of respective centers of adjacent rollers in the chain increase. The occurrence of the stretch in the chain may cause loosening of the chain and cause defects such as tooth-skipping. Moreover, the occurrence of the stretch in the chain may cause a meshing position, which is a position at which the chain and a sprocket mesh with each other, to move from a tooth bottom to a tooth top of the sprocket. Such movement may increase shearing stress generated in teeth of the sprocket and cause defects such as loss of teeth. Typically, slide wear generated at the rotary joint portion is suppressed with lubricating oil supplied to the rotary joint portion of the chain in the case of drive of the chain. However, such measure merely suppresses progress of the wear generated at the rotary joint portion and cannot prevent the stretch of the chain over years caused by long-term drive of the chain.
In view of the foregoing, there has hitherto been known a chain stretch detection device having the following configuration, which is used as a measure for preventing, in advance, the defects such as tooth-skipping or loss of teeth. Through utilization of the phenomenon in which the meshing position at which the chain and the sprocket mesh with each other moves from the tooth bottom to the tooth top when the stretch occurs in the chain, the meshing position or a meshing height is measured, and an amount of the stretch of the chain is specified based on the meshing position or the meshing height having been measured. Then, a state of the stretch in the chain is grasped based on the specified stretch amount of the chain (see, for example, Patent Literature 1).